The present invention relates to a developing solution for a photosensitive resin, which permits markedly improving the residual film characteristics and the resolution characteristics in the formation of a photosensitive polyimide pattern used as, for example, an insulating member, a protective film member, a liquid crystal element member or an optical element member. The present invention also relates to a pattern forming method that permits obtaining a polyimide film pattern excellent in resolution, heat resistance and adhesivity and used for forming an insulating film, a protective film, an .alpha.-ray shielding film or an optical waveguide of various electronic parts. Further, the present invention relates to an electronic part prepared by employing the particular pattern forming method.
In the ordinary semiconductor device comprising a semiconductor substrate, the surface of the semiconductor substrate bearing semiconductor elements is generally covered with a protective film called a passivation film for the purpose of improving the reliability of the semiconductor device by protecting the semiconductor elements from any influence by the outer environment. As a material for this passivation film, polyimide resin is extensively employed in view of its excellent properties such as electric properties, e.g. insulating characteristics, radiation resistance, environmental stability and heat resistance. Furthermore, this polyimide is widely employed in a semiconductor device as an .alpha.-ray shielding film, or as an interlayer insulating film for a multilayered wiring structure or for a multilayered element structure (a multichip module).
This polyimide can be easily formed into a film from polyamic acid which is a precursor for the polyimide. Namely, a varnish of polyamic acid is coated on the surface of a predetermined substrate and then the coated layer is heated to form a film of polyamic acid, which is then heat-treated at a high temperature thereby to cause a cyclodehydration reaction of the polyamic acid to take place, thus curing the polyamic acid film and forming a polyimide film. It is possible, with the employment of this method, to easily form a film of polyimide in spite of the fact that polyimide can be hardly worked since it is not or hardly soluble to most of organic solvents and is high in softening point. Therefore, the aforementioned method has been widely adopted in the formation of polyimide film.
Meanwhile, in the manufacture of a semiconductor device, various workings, for example, for forming a through-hole in a multilayered wiring structure or for forming a pad for effecting an electric connection with an external lead are required. In order to carry out these workings, a polyimide film formed as a protective film (passivation film) or an interlayer insulating film as explained above is required to be patterned thereby to form holes or grooves of predetermined patterns. Generally, the patterning of polyimide film is performed by making use of PEP (photo-engraving process) using a photoresist. Namely, after a polyimide film is formed on the surface of a semiconductor substrate bearing a semiconductor element thereon by the aforementioned method, a photoresist film is formed on the surface of the polyimide film and then subjected to a light exposure process followed by a development process thereby forming a resist pattern. Then, the underlying polyimide film is selectively etched by using this resist pattern as an etching mask thereby to form a polyimide protective film or interlayer insulating film having a prescribed pattern.
However, the aforementioned method of forming a polyimide pattern requires two independent steps, i.e. a step of forming a polyimide film and a step of the PEP, making the method troublesome to carry out.
With a view to solve the aforementioned drawback in carrying out the method, a method of patterning a polyimide film without employing the PEP has been demanded. In response to such a demand, a resin composition comprising a polyimide precursor has been proposed.
For example, each of Japanese Patent Disclosure (Kokai) No. 52-13315 and Japanese Patent Disclosure No. 62-135824 discloses a composition prepared by adding an o-quinone diazide compound as a photosensitizer to a polyimide precursor as a photosensitive resin composition having a photosensitivity of positive type.
Each of these photosensitive resin compositions is prepared by adding a photosensitizer (inhibitor of a photosensitive dissolution) to a polyimide precursor, i.e., polyamic acid. For forming a polyimide film pattern by using such a photosensitive resin composition, a substrate surface is coated with a solution of the photosensitive resin composition, followed by drying the coating to form a resin layer and subsequently forming a pattern by light exposure and developing treatments. Further, a heat treatment is applied to the resin layer so as to cure, or imidize, the polyimide precursor by a dehydration-cylization reaction.
These photosensitive resin compositions can be developed with an alkaline developing solution. It was customary to use as the alkaline developing solution for the photosensitive polyimide an aqueous solution of an inorganic alkaline material such as sodium hydroxide, potassium hydroxide, sodium carbonate and potassium carbonate and an organic alkaline material such as tetramethyl ammonium hydroxide, choline, triethyl amine, ethanol amine, or diethyl ethanol amine, said aqueous solution being widely used as an alkaline developing solution for a photoresist material.
These inorganic and organic alkaline materials are compounds having a very strong basicity. Specifically, these inorganic and organic alkaline materials have a base dissociation index pKb not larger than 4.5 (i.e., acid dissociation index pKa of the proton complex not smaller than 9.5). Since an aqueous solution of these inorganic and organic alkaline materials dissolves the photosensitive polyimide very rapidly, the aqueous solution is used as a dilute solution having a concentration not higher than 0.05 mol/L (liter).
However, use of the conventional developing solution for the photosensitive polyimide gives rise to serious problems that the resolution characteristics and the residual film characteristics are very poor.